There are cases in which, for safety reasons, screws or nuts must be secured in a manner that rules out the loosening of said screw or nut. So far, it is standard in such cases to incapacitate the screw drive that serves for the application of a tool after the screw has been fastened. This can happen, for example, through boring or smashing the plug that then prevents the application of a tool into the drive design. There are also tear-off screws on which a predetermined breaking point is provided, which gives way upon attaining the required torque. All these systems have the disadvantage that they require additional measures and, in part, they must be handled pedantically. Moreover, tear-off screws pose the problem of corrosion.
A screw is already known, in which the rotation drive is provided with three triangle-shaped contact surfaces in a recess of the screw head, respectively located in a radial plane. A special screwdriver can be used to drive in these screws, whereas, in the reversed direction of rotation, the tool does not drive the screw (DE 3403063 A1).
The objective of the invention is to create a possibility of designing connections to be established by means of screws, so that they can only be loosened with difficulty or if destroyed.
To meet this objective, the invention proposes a drive design with the features of claim 1. The invention also proposes a drive design for an accompanying tool. Further embodiments of the invention are the object of dependent claims.
This means that one can rotate the fastening element in one direction, naturally the drive-in direction; however, torque transmission is not possible in the reversed direction of rotation. This can be achieved, for example, in that the transition surfaces extend so obliquely that the force applied in the direction of rotation produces a force component that presses the tool out of the recess.
In a further embodiment of the invention, the transition surfaces that serve for guiding the tool in the rotation drive can be formed as inwardly aligned inside surfaces. The tool therefore engages with them with a radial outside part.
It is, however, also possible and within the scope of the invention that the transition surfaces are formed as radial outside surfaces. In this case, the tool grips on the outside surfaces.
Particularly, it can be made possible that the drive surfaces and the transition surfaces are formed in a recess originating from the end face of the fastening element. In this case, it relates to a screw, for example, with which the end face of the screw head facilitates the access to the recess. In this case, the screw head of such a screw therefore contains a recess in which at least two drive surfaces suitable for the transmission of torque are arranged for rotating with a tool in a direction of rotation, between which transition surfaces are disposed, which, upon engagement of the tool in the other direction, cause an axial displacement of the tool out of the recess.
In order to cause a particularly effective force transmission when driving in the screw, in accordance with the invention it can be provided that the drive surfaces are disposed in radial planes. A certain deviation from the radial plane is also possible, so that, in a further embodiment through the drive design, the invention proposes that in a cross-section extending transversely to the axis of the screw the drive surfaces lie respectively on a radius or extend parallel to one another.
With an even number of drive surfaces, it can be provided therefore in accordance with the invention that the drive surfaces in a cross-section extending transversely to the axis of the screw through the drive design lie on a diameter or extend parallel to one another and on a diameter.
In a longitudinal section through the fastening element, the drive surfaces extend preferably parallel to the axis or in the axis.
In a further embodiment of the invention, it can be made possible that the drive surfaces form a drive edge that extends obliquely to the axis, whereby the drive edges of all drive surfaces diverge in the direction of the end face of the screw head.
For example, the drive edges can extend between the wall of the recess and the bottom of the recess.
In accordance with the invention, it can be made possible that the transition surfaces are formed as wedged surfaces, whereby the wedge is oriented such that, by introducing torque in the reverse screwing direction, a force component is produced out of the recess.
Particularly, it can be made possible that the transition surfaces extend between the drive edge of a drive surface of the drive surface connection with the bottom of the recess and the wall of the recess.
It has turned out to be particularly practical if the drive design comprises an odd number of drive surfaces and transition surfaces.
In a further embodiment of the invention, it can be made possible that the recess at least partially features a decreasing cross-section in the direction from the end face away from the screw head. This is reasonable particularly in the case of countersunk screws.
In yet a further embodiment of the invention, it can be made possible that the recess is formed at least partially cylindrical. Such a cylindrical recess can also serve to improve the guidance of a tool better than it would in a recess that decreases in cross-section right from the beginning.
In accordance with the invention, it can be made possible that the recess runs in a pointed tip.
However, it is also possible and proposed by the invention that the recess features a bottom that is formed preferably plane.
In accordance with the invention, it can be made possible that the wall of the recess at least in an axial section lies on a cylindrical jacket surface, particularly in a section originating from the end face of the screw head.
The screwdriver also proposed by the invention features a free end the drive design of which is formed complementarily to the drive design of the screw head.
Further features, details and advantages of the invention are derivable from the claims and abstract, whereby the wording in both is based on reference to the contents of the description, following description of preferred embodiments of the invention as well as of the drawing. Shown hereby: